ASE swept source with self-tracking filter for OCT medical imaging

ABSTRACT

An integrated swept wavelength tunable optical source uses a narrowband filtered broadband signal with an optical amplifier and self-tracking filter. This source comprises a micro optical bench, a source for generating broadband light, a tunable Fabry Perot filter, installed on the bench, for spectrally filtering the broadband light from the broadband source to generate a narrowband tunable signal, an amplifier, installed on the bench, for amplifying the tunable signal. The self-tracking arrangement is used where a single tunable filter both generates the narrowband signal and spectrally filters the amplified signal. In some examples, two-stage amplification is provided. The use of a single bench implementation yields a low cost high performance system. For example, polarization control between components is no longer necessary.

RELATED APPLICATIONS

This application is a Continuation-in-Part (CIP) of U.S. applicationSer. No. 12/553,295, filed on Sep. 3, 2009, which is incorporated hereinby reference in its entirety.

BACKGROUND OF THE INVENTION

Optical coherence analysis relies on the use of the interferencephenomena between a reference wave and an experimental wave or betweentwo parts of an experimental wave to measure distances and thicknesses,and calculate indices of refraction of a sample. Optical CoherenceTomography (OCT) is one example technology that is used to performusually high-resolution cross sectional imaging. It is often applied toimaging biological tissue structures, for example, on microscopic scalesin real time. Optical waves are reflected from an object or sample and acomputer produces images of cross sections of the object by usinginformation on how the waves are changed upon reflection.

The original OCT imaging technique was time-domain OCT (TD-OCT), whichused a movable reference mirror in a Michelson interferometerarrangement. More recently Fourier domain OCT (FD-OCT) has beendeveloped. Two related FD-OCT techniques are time encoded and spectrumencoded OCT. These Fourier domain techniques use either a wavelengthswept source and a single detector, sometimes referred to astime-encoded FD-OCT (TEFD-OCT) or swept source OCT, or, alternatively, abroadband source and spectrally resolving detector system, sometimesreferred to as spectrum-encoded FD-OCT or SEFD-OCT. These three OCTtechniques parallel the three spectroscopy approaches implemented byFourier transform spectrometers, tunable laser spectrometers, anddispersive grating with detector array spectrometers.

These various OCT techniques offer different performancecharacteristics. FD-OCT has advantages over time domain OCT (TD-OCT) inspeed and signal-to-noise ratio (SNR). Of the two Fourier Domain OCTtechniques, swept-source OCT or TEFD-OCT has distinct advantages overSEFD-OCT because of its capability of balanced and polarizationdiversity detection; it has advantages as well for imaging in wavelengthregions where inexpensive and fast detector arrays are not available.

TEFD-OCT or swept source OCT has advantages over SEFD-OCT in someadditional respects. The spectral components are not encoded by spatialseparation, which requires bulky grating arrangements, but they areencoded in time, which can utilize compact swept wavelength sources. Thespectrum is either filtered or generated in successive frequency stepsof the swept source and is reconstructed before Fourier-transformation.Using the frequency scanning swept source the optical configurationbecomes less complex and more compact, but the critical performancecharacteristics now reside in the source and especially its tuning speedand accuracy.

The swept sources for TEFD-OCT systems have been typically tunablelasers. The advantages of tunable lasers include high spectralbrightness and relatively simple optical designs. The typical tunablelaser is constructed from a gain medium, such as a semiconductor opticalamplifier (SOA), placed inside an optical laser cavity which includes anintracavity tunable filter, such as a rotating grating, fixed gratingwith a rotating mirror, or a Fabry-Perot tunable filter. Currently, someof the highest speed TEFD-OCT lasers are based on the laser designsdescribed in U.S. Pat. No. 7,415,049 B1, entitled “Laser with TiltedMulti Spatial Mode Resonator Tuning Element”, by D. Flanders, M.Kuznetsov and W. Atia. These highly integrated designs allow for a shortlaser cavity that keeps the round-trip optical travel times within thelaser cavity short, so that the laser is fundamentally capable of highspeed tuning Secondly, the use of micro-electro-mechanical system (MEMS)Fabry-Perot tunable filters combines the capability for wide spectralscan bands with the low-mass high mechanical resonant frequencydeflectable MEMS membranes that can be tuned rapidly.

Another swept laser source for OCT is the Frequency Domain ModelockedLaser (FDML) as described in U.S. Pat. No. 7,414,779 B2. FDML lasers usesemiconductor optical amplifiers in a very long, kilometer or more,fiber ring laser cavities that require polarization control and activelength stabilization.

The use of laser-based swept sources, however, does have problems. Theinstantaneous laser emission is characterized by one or morelongitudinal laser cavity modes that simultaneously lase within thepassband of the laser's tunable filter. Then as the laser tunes, thepower within these modes shifts between the modes and to new cavitymodes that see gain as the tunable filter passband shifts. Thismulti-mode spectral structure of the laser emission increases relativeintensity noise (RIN), which degrades performance of OCT systems.Another problem is that tunable lasers using ubiquitous semiconductorgain media generally only tune well in one direction, i.e., to longerwavelengths. This is due to a nonlinear asymmetric gain effect insemiconductors that is often called the Bogatov effect. With an opticalsignal in a semiconductor at a given wavelength, optical waves at longerwavelengths will experience slightly higher optical gain, while opticalwaves at shorter wavelengths will experience slightly lower opticalgain. Such asymmetric nonlinear gain distribution creates a preferencefor dynamic tuning in the longer wavelength direction, where opticalgain is slightly higher, while impeding tuning in the shorter wavelengthdirection.

Another limitation of tunable laser sources is that their tuning speedis limited by the round-trip time of the laser cavity. Shortening thelaser cavity allows for faster scan speeds, but increases thelongitudinal mode spacing and thus reduces the number of modes that canlase within the filter linewidth. The reduced number of lasing modeincreases the RIN, and can ultimately lead to mode-hopping. On the otherhand, one can increase the filter linewidth to allow a larger number ofmodes to lase for a lower laser RIN, but this increased laser linewidthresults in shorter coherence length that may not be adequate for imagingdeeper objects. Potential maximum imaging depth of a swept source OCTsystem is given by one half the coherence length of the system source,where the coherence length is inversely proportional to the dynamiclinewidth of the swept source. Moreover, for a given cavity length andfilter linewidth, increasing scan speeds will reduce coherence lengthand ultimately cause the source to cease lasing.

Another class of swept sources that have the potential to avoid theinherent drawbacks of tunable lasers is filtered amplified spontaneousemission (ASE) sources that combine a broadband light source, typicallya source that generates light by ASE, followed by tunable filters andoptical amplifiers. Some of the highest speed devices based on thisconfiguration are described in U.S. Pat. No. 7,061,618 B2, entitledIntegrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, andM. Kuznetsov, which describes spectroscopy engines for diffusereflectance spectroscopy and other spectroscopic applications such asOCT. A number of variants of the filtered ASE swept source are describedincluding amplified versions and versions with tracking filters.

More recently Eigenwillig, et al. have proposed a variant configurationof the filtered ASE source in an article entitled “Wavelength swept ASEsource”, Conference Title: Optical Coherence Tomography and CoherenceTechniques IV, Munich, Germany, Proc. SPIE 7372, 737200 (Jul. 13, 2009).The article describes a source with an SOA functioning both as an ASEsource and first amplification stage. Two Fabry-Perot tunable filtersare used in a primary-tracking filter arrangement, which are followed bya second SOA amplification stage.

Such swept filtered ASE sources, while typically more complex opticallythan some lasers, do provide some performance advantages. For example,they do not have laser optical cavities and thus do not have the lasertuning speed limitations imposed by the finite cavity roundtrip time.Moreover, the lack of the laser cavity avoids the problems associatedwith the discrete longitudinal laser cavity modes.

SUMMARY OF THE INVENTION

Generally, in the ASE sources, one or more SOA amplification stages arerequired. This is because the seed broadband ASE source produces lightover a wide spectral range. The tunable filter then rejects most of thatlight allowing only light in the typically narrow spectral passband ofthe tunable filter to be transmitted. As a result, this narrowbandsignal is relatively weak in power since most of the light generated bythe seed ASE source is rejected and therefore not used. This problemgenerally does not arise in tunable laser sources since the SOA islocated within the laser cavity and generates light predominantly onlyat the emission wavelengths within the tunable filter passband.

The need for one or more SOA amplification stages, for mostapplications, necessitates the requirement for one or more trackingfilters. This arises because when the relatively weak signal from theprimary filter is amplified, there is a large contribution of additivebroadband optical noise outside the passband of the primary tunablefilter, with the added noise power that is comparable to the signalpower itself The tracking filter is thus required in many instances toattenuate this noise outside the passband of the primary filter.

The use of multiple tunable filters creates its own problems, however.The second filter increases the parts count for the system.Additionally, in operation, the filters must be well synchronized. Theirpassbands must be precisely aligned in wavelength as both filters arebeing tuned, otherwise the power level of the output signal light willbe severely degraded. Moreover, high-speed scanning is often desirablein OCT systems. This further complicates the filter alignment task,requiring very precise high-speed dynamic filter tuning synchronization.

In order to ease the requirements for the filter synchronization, thepassband of the tracking filter can be selected to be much wider thanthe primary filter. In one example the primary filter may have a widthof tens of gigahertz (GHz) whereas the tracking filter may have a widthof about 100 GHz. This reduces the accuracy with which the trackingfilter must track the operation of the primary filter. Both filter orderarrangements are possible: (i) the narrow filter can filter the originalbroadband seed ASE, while the wider tracking filter follows anamplification stage; also (ii) the wider filter can serve to filter theoriginal broadband seed ASE, while the narrower filter then serves tofilter ASE of an amplification stage. There is a downside, however, tothis narrow-wide filter configuration. The relatively wide trackingfilter allows excessive ASE emissions in a band around the signal light.Finally, the filters can drift differently over time, leading to a needto calibrate the filter tuning system or to a complex filter tuningcontrol loop.

Present invention is directed to a swept source that can address some orall of these problems. It uses a tunable filter in a self-trackingconfiguration. That is, light is passed through the same filter multipletimes. This has the effect of applying the filter transfer function oneach pass through the tunable filter. This successively narrows thespectral width of the signal, which is desired for narrow linewidth andthus long coherence length operation required for deeper imaging. At thesame time, using the filter in this self tracking configuration avoidsthe need for the complex task of synchronizing tuning of the twofilters.

In general according to one aspect, the invention features a sweptoptical source, comprising: a tunable filter for spectrally filteringlight from a source to generate a tunable optical signal that isspectrally tuned over a scanband and an optical amplifier for amplifyingthe tunable optical signal with the amplified tunable optical signalbeing filtered by the tunable filter.

In embodiments, the source comprises a broadband source that generateslight over the scanband and in implementations is an amplifiedspontaneous emission source that generates light over the scanband.

In embodiments the tunable filter is a Fabry-Perot tunable filter suchas a micro electro mechanical system Fabry Perot tunable filter. Itspassband is preferably less than 20 GigaHertz (FWHM) and often less than5 GigaHertz.

In other embodiments the tunable filter is based on gratings, acoustooptic tunable filters or tilt tuned interference/etalon filters.

A tuning controller is preferably used to drive the tunable filter totune over the scanband at a speeds greater than 10 kiloHertz (kHz) andpreferably greater than 100 kHz for some applications.

In the current embodiment, the optical amplifier comprises asemiconductor optical amplifier. In some cases, a reflector is used forreflecting the tunable optical signal to pass through the opticalamplifier a second time and back to the tunable filter. In other cases,a loop including the optical amplifier is used for directing the tunableoptical signal back to the tunable filter.

A polarization rotation system is used for rotating a polarization ofthe tunable optical signal between being filtered by the tunable filter;this system includes a non-reciprocal polarization rotation element.

In some embodiments, the filtering by the tunable filter of the tunableoptical signal is co-directional with the filtering of the amplifiedtunable optical signal by the tunable filter, whereas in otherembodiments, the filtering by the tunable filter of the tunable opticalsignal is contra-directional with the filtering of the amplified tunableoptical signal by the tunable filter.

In general according to another aspect, the invention features a methodfor generating a tunable optical signal, comprising spectrally filteringlight to generate a tunable optical signal with a tunable filter that isspectrally tuned over a scanband, amplifying the tunable optical signalto generate an amplified tunable optical signal, and spectrallyfiltering the amplified tunable optical signal with the same tunablefilter.

In general according to still another aspect, the invention featurestunable amplifier comprising a tunable filter for spectrally filteringlight to generate a tunable optical signal that is spectrally tunableover a spectral band and an optical amplifier for amplifying the tunableoptical signal with the amplified tunable optical signal being filteredby the tunable filter.

In general according to still another aspect, the invention features aswept optical source comprising a tunable filter for spectrallyfiltering light from a source to generate a tunable optical signal thatis spectrally tuned over a scanband, an optical amplifier for amplifyingthe tunable optical signal in a first pass, and a reflector forreflecting the tunable optical signal to pass through the opticalamplifier in a second pass.

In general according to still another aspect, the invention features anoptical coherence analysis system comprising a swept source including atunable filter for spectrally filtering light from a source to generatea tunable optical signal that is spectrally tuned over a scanband and anoptical amplifier for amplifying the tunable optical signal with theamplified tunable optical signal being filtered by the tunable filter. AMichelson interferometer divides the amplified tunable optical signalbetween a reference arm and a sample arm and generates an interferencesignal by combining the optical signal from the reference arm and thesample arm. Finally, a detector system detects the interference signal.

The above and other features of the invention including various noveldetails of construction and combinations of parts, and other advantages,will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. It will beunderstood that the particular method and device embodying the inventionare shown by way of illustration and not as a limitation of theinvention. The principles and features of this invention may be employedin various and numerous embodiments without departing from the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, reference characters refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis has instead been placed upon illustrating theprinciples of the invention. Of the drawings:

FIG. 1 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda double-pass reflective amplification stage according to a firstembodiment of the present invention;

FIG. 2 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda double-pass reflective amplification stage using an isolator accordingto a second embodiment of the present invention;

FIG. 3 is a scale plan view of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda double-pass reflective amplification stage using an isolator accordingto a third embodiment of the present invention;

FIG. 4 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversitywith a loop amplification stage according to a fourth embodiment of thepresent invention;

FIGS. 5A and B are scale top plan and perspective views of a sweptoptical source with a contra-directional self-tracking filter usingpolarization diversity with a loop amplification stage according tofifth embodiment;

FIG. 6 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda loop amplification stage with dual amplifiers according to a sixthembodiment;

FIG. 7 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda loop amplification stage with an additional, tracking filter accordingto a seventh embodiment of the present invention;

FIG. 8 is a block diagram of a swept optical source with acontra-directional self-tracking filter using polarization diversity anda loop amplification stage and an output amplification stage accordingto an eighth embodiment of the present invention;

FIG. 9 is a block diagram of a swept optical source with aco-directional self-tracking filter using polarization diversity and adouble-pass loop amplification stage according to a ninth embodiment ofthe present invention;

FIG. 10 is a block diagram of a swept optical source with aco-directional self-tracking filter using polarization diversity and aloop amplification stage according to a tenth embodiment of the presentinvention;

FIG. 11 is a block diagram of a swept optical source with aco-directional self-tracking filter in a filter loop using polarizationdiversity and a double pass reflective amplification stage according toan eleventh embodiment of the present invention;

FIG. 12 is a block diagram of a swept optical source with aco-directional self-tracking filter in a filter loop using polarizationdiversity and a double pass reflective amplification stage according toa twelfth embodiment of the present invention;

FIG. 13 is a block diagram of a swept optical source with aco-directional self-tracking filter in a double pass amplifier—filterloop using polarization diversity and an additional amplification stagewith a tracking filter according to a thirteenth embodiment of thepresent invention;

FIG. 14 is a block diagram of a swept optical source with aco-directional dual-pass self-tracking filter using polarizationdiversity with dual loops according to a fourteenth embodiment of thepresent invention;

FIG. 15 is a block diagram of a swept optical source with aco-directional self-tracking filter in an amplified dual-pass loop usingpolarization diversity and a second output loop according to a fifteenthembodiment of the present invention; and

FIG. 16 is a block diagram of an OCT system using the inventive sweptsources;

FIG. 17 is a block diagram of a spectroscopy system using the inventiveswept sources; and

FIG. 18 is a plot of power as a function of wavelength (nanometers)showing the relationship between the spectral extent of the broadbandoptical signal, the scanband, and the tunable optical signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description similar components in the differentembodiments are given the same or similar reference numerals to indicatesimilar construction and functionality.

FIG. 1 shows a first embodiment swept optical source 100 a withcontra-directional self-tracking filter using polarization diversity anda reflective, double-pass amplification stage, which has beenconstructed according to the principles of the present invention.

Preferably, the source 100 a is entirely implemented on a single bench110 with free space optics coupling the optical beams between theoptical elements. In the coordinate system of the figure, the benchextends in the x-z plane and the y-axis extends vertically from theplane of the bench.

In other examples, many or most of the optical elements are implementedin common on a single optical bench. In yet other examples, two or moreseparate optical benches are used to implement the swept source, placingsome optical components on one bench and other optical components onanother bench, with free-space or optical fiber connections betweendifferent benches.

The use of the optical bench and free space optics in this and the otherembodiments is important for at least two reasons. In this and the otherdisclosed embodiments, the self-tracking filter is implemented using apolarization diversity scheme. The use of the single optical bench easesthe assembly requirements to ensure that the orthogonal polarizationspass through the self-tracking filter. When using optical fiberconnections, it can be difficult to ensure proper polarization alignmentof the different optical signals in the system. Moreover, using standardsingle-mode fiber causes a randomized polarization state of the outputlight requiring polarization controllers between elements. Additionally,the use of the self tracking filter in a high speed tuning sourcerequires that there is only a small time delay between the separatetransits of the optical signals through the typically rapidly-tunedself-tracking filter. As the tuning speed of the system increases, onlysmaller and smaller time delays are tolerable. The implementation on asingle, e.g. micro-optical, bench with little or no fiber links ensuresthat the system has very short optical propagation time delays and isable to operate at very high tuning speeds.

The bench 110 is termed a micro-optical bench and is preferably lessthan 10 millimeters (mm) in width and about 20 mm in length or less.This size enables the bench 110 to be installed in a standard, or nearstandard-sized, butterfly or DIP (dual inline pin) hermetic package. Inone implementation, the bench 110 is fabricated from aluminum nitride. Athermoelectric cooler is disposed between the bench 110 and the package(attached/solder bonded both to the backside of the bench and innerbottom panel of the package) to control the temperature of the bench110.

The swept source 100 a comprises a broadband source 112 that generates abroadband optical signal. In general, the broadband signal ischaracterized by a continuous spectrum that extends in wavelength overat least 40 nanometers (nm) of bandwidth, full width half maximum(FWHM). Typically, the continuous spectrum extends over at least 70 nmand preferably over 100 nm or more.

In the current embodiments described herein, the proposed filtered sweptsources start with a broadband amplified spontaneous emission (ASE)source. Semiconductor optical amplifiers, or SOAs, are effective sourcesof such broadband ASE. SOAs are typically configured with an opticalinput and an optical output for input light amplification. With no inputlight, only ASE appears at the SOA output. Sometimes superluminescentlight emitting diodes, or SLEDs, are used as broadband ASE sources.SLEDs are typically SOAs configured with an optical access to the outputASE signal and no optical access to the SOA input.

In the preferred embodiment, the broadband source 112 is an electricallypumped semiconductor diode chip gain medium that is bonded or attachedto a bench 110. Such devices generate the broadband light by ASE. Commonexamples of the source 112 include superluminescent light emittingdiodes (SLED) and semiconductor optical amplifiers (SOA) implemented ina SLED configuration, i.e. functioning to generate ASE light. Suchbroadband ASE sources are often fabricated, for example, near 840nanometer (nm), 1060 nm, or 1310 nm center wavelengths.

The material system of the chip gain medium is selected based on thedesired spectral operating range. Common material systems are based onIII-V semiconductor materials, including binary materials, such as GaN,GaAs, InP, GaSb, InAs, as well as ternary, quaternary, and pentenaryalloys, such as InGaN, InAlGaN, InGaP, AlGaAs, InGaAs, GaInNAs,GaInNAsSb, AlInGaAs, InGaAsP, AlGaAsSb, AlGaInAsSb, AlAsSb, InGaSb,InAsSb, and InGaAsSb. Collectively, these material systems supportoperating wavelengths from about 400 nm to 2000 nm, including longerwavelength ranges extending into multiple micrometer wavelengths.Semiconductor quantum well and quantum dot gain regions are typicallyused to obtain especially wide gain bandwidths. Currently, edge-emittingchips are used although vertical cavity surface emitting laser (VCSEL)chips can be used in different implementations.

The use of a semiconductor chip gain medium for the source 112 hasadvantages in terms of system integration since it can be bonded to asubmount that in turn is directly bonded to a bench 110. Other possiblegain media can be used in other implementations, however. In theseexamples, the broadband signal is typically transmitted via opticalfiber to the bench 110. Such examples include solid state gain media,such as rare-earth (e.g., Yb, Er, Tm) doped crystals, bulk glass,waveguides or optical fiber.

In these examples, the output facets of the chips or gainwaveguides/fibers are antireflection coated, and possibly angled, sothat output ASE signal is not fed back into the gain medium and the gainmedium does not lase but instead generates broadband light via ASE.Sometimes two co-directional or contra-directional optical passesthrough the SOA amplifier are used. Presence of repetitive periodicoptical feedback into the gain medium, such as from an optical cavity,would result in device lasing and would destroy the required operatingregime of the filtered ASE swept source, for example by making thedevice non tunable. In all of the proposed ASE swept sourceconfigurations here, great effort, such as by use of isolators, isexpended to prevent the formation of optical cavities about the gainelements and thus to prevent device from lasing, which would destroy thedevice operation.

The broadband source 112 in this and the other embodiments is notlimited to only sources that generate light via ASE. In otherembodiments, the broadband source 112 is implemented as a supercontinuumsource based on nonlinear fiber, a Raman light source, or a parametriclight source, for example.

The broadband optical signal 114 from the broadband source 112 istypically highly polarized. This is a characteristic of light generatedby quantum-well gain semiconductor chips. In the most commonquantum-well SOA or SLED ASE source, light is polarized in the plane ofthe optical wafer or the chip, commonly referred to as TE polarization.In the illustrated embodiment, the broadband optical signal 114 ispolarized in a direction that is parallel to the surface of the bench110. This is illustrated in the series of inset diagrams that show theangle of polarization P from the perspective of the optical signal 114,looking into the beam, along its optical axis of propagation.

Other SOA or SLED chips have light polarized perpendicular to the planeof the chip, commonly referred to as TM polarization. If such chips areused, broadband optical signal 114 will be polarized in a direction thatis perpendicular to the surface of the bench 110. Either polarizationtype, TE or TM, source or amplifier can be used in the filtered sourcearrangements, as long as proper polarization orientation and rotation isarranged along the optical beam path.

A first halfwave plate 210 rotates the polarization of the broadbandsignal by 45 degrees. A polarizer 116 filters the rotated broadbandsignal 114 from the broadband source 112. The polarizer is oriented tobe parallel to the predominant polarization of the broadband signal 112as rotated by the halfwave plate 210. As a result, the broadband signal114 passes through the polarizer 116.

A first broadband non-reciprocal rotator, or Faraday rotator, 118rotates the polarization of the broadband signal back 45° so that thebeam again has a horizontal polarization.

The broadband signal from the Faraday rotator 118 is transmitted to apolarization beam splitter (PBS) 120. The orientation of the PBS 120 isconfigured to transmit the broadband signal 114, at the horizontalpolarization angle in the illustrated implementation. Thus the broadbandsignal 114 is transmitted directly through the PBS 120. Typically, thehighly polarized output characteristic of semiconductor gain mediaensures a high transmission efficiency.

In other implementations, the polarization beam splitter 176 isimplemented as a polarizing coating or a birefringent walk-offpolarizer.

In still another implementation, a circulator is used instead of the PBS120. Further a simple beam splitter or coupler is used in otherexamples. This is considered as suboptimal due to the concomitantoptical losses, however.

A self-tracking tunable filter 150 in this and the other embodimentsfunctions first as a tunable bandpass filter to convert the broadbandsignal 114 to a narrow band tunable signal 154. In a current embodiment,the passband of the self-tracking tunable filter 150 has a full widthhalf maximum (FWHM) bandwidth of less than 20 or 10 GigaHertz (GHz), andis preferably 5 GHz or less. For spectroscopy this relatively narrowpassband yields high spectral resolution. For optical coherencetomography, this high spectral resolution implies a long coherencelength of the source and therefore enables imaging deeper into samples,for example deeper than 5 mm. In lower performance applications, forexample OCT imaging less than 1 mm deep into samples, broader FWHMpassbands are sometimes appropriate, such as passbands of about 200 GHzor less.

the self-tracking tunable filter 150 in this and the other embodimentsis preferably a Fabry-Perot tunable filter that is fabricated usingmicro-electro-mechanical systems (MEMS) technology and is attached, suchas directly solder bonded, to the bench 110. Currently, the filter 150is manufactured as described in U.S. Pat. Nos. 6,608,711 or 6,373,632,which are incorporated herein by this reference. A curved-flat resonatorstructure is used in which a generally flat mirror and an opposed curvedmirror define a filter optical cavity, the optical length of which ismodulated by electrostatic deflection of at least one of the mirrors.

In the current embodiment, the filter is installed perpendicular to theoptical axis and is oriented with the fixed mirror on the side nearestthe amplification stage. The membrane side is more sensitive to photonpressure. Thus, orienting the filter such that the post amplificationside hits the fixed mirror first is preferable.

In this and the other embodiments described hereinbelow, other tunablefilters and spectral filter technologies are possible. In someimplementations, grating based filters are used including rotatinggratings and gratings with rotating mirrors. Further, in still otherimplementations other Fabry-Perot filter technologies are used includingpiezoelectrically and thermally tuned Fabry-Perot filters. In stillfurther examples angle-tuned, including spinning Fabry-Perot etalons andangle-tuned, including spinning, interference filters are used. Thinfilm interference filter is a group of several coupled FP filters formedby depositing thin-film reflective coatings with intervening spacersonto a substrate; these are used widely for wavelength divisionmultiplexing (WDM) channel selection applications. Acousto-optic tunablefilters (AOTF) are another alternative.

Tuning controller 152 in this and the other embodiments drives theself-tracking tunable filter 150. Preferably, the tuning controller 152tunes the passband over the scanband extending over the gain band ofbroadband source 112. In the illustrated embodiment, this is achieved bymodulating the electrostatic drive voltage applied to the MEMS tunablefilter. Particularly, passband is tuned over a scanband that covers thecontinuous spectrum associated with broadband optical signal 114 andthus tunes over a spectral scan band of about 70 nm to 100 nm orgreater. Other wavelength tuning ranges are greater than (i) 10 nm, (ii)40 nm, (iii) 80 nm.

The swept optical source 100 a and the other embodiments discussedhereinbelow are generally intended for high speed tuning to generatetunable optical signal that scans over the scanband at speeds greaterthan 10 kiloHertz (kHz). In current embodiments, the swept opticalsource 100 a tunes at speeds greater than 50 or 100 kHz. In very highspeed embodiments, the swept optical source 100 a tunes at speedsgreater than 200 or 500 kHz, or faster.

The tuning controller 152 provides a tuning voltage function that sweepsthe passband optical frequency across the tuning band, preferably withoptical frequency varying linearly with time. The tuning controller alsopreferably provides bidirectional tunable filter sweeps, in the up anddown wavelength tuning directions in a sawtooth fashion. Alternatively,the tuning controller provides unidirectional wavelength tuning sweeps,for example in the up tuning wavelength direction, with fast sweepretrace for a high duty cycle linear in time frequency sweep.

The tuning speed provided by the tuning controller is also expressed inwavelength per unit time. In one example, for an approximately 110 nmtuning range or scan band and 100 kHz scan rate, assuming 60% duty cyclefor substantially linear up-tuning, the peak sweep speed would be 110nm*100 kHz/0.60=18,300 nm/msec=18.3 nm/μsec. In another example, for anapproximately 90 nm tuning range and 50 kHz scan rate, assuming a 50%duty cycle for substantially linear up-tuning, the peak sweep speed is90 nm*50 kHz/0.50=9,000 nm/msec=9.0 nm/μsec. In a smaller scan bandexample having an approximately 30 nm tuning range and 2 kHz scan rate,assuming a 80% duty cycle for substantially linear up-tuning, the peaksweep speed would be 30 nm*2 kHz/0.80=75 nm/msec=0.075 nm/μsec

Thus, in terms of scan rates, in the preferred embodiments describedherein, the sweep speeds are greater than 0.05 nm/μsec, and preferablygreater than 5 nm/μsec. In still higher speed applications, the scanrates are higher than 10 nm/μsec.

Light that is outside the passband, in the case of a Fabry-Perot tunablefilter, is reflected. This reflected light, however, does not form alaser cavity with the current configuration around the broadband source112. Any back-reflected light from the tunable filter 150 is furtherrotated by the non-reciprocal, or Faraday, rotator 118 to have apolarization that is now perpendicular to the axis of the polarizer 116due to the non-reciprocal operation of the Faraday rotator 118. Thereflected light is thus stopped by the polarizer 116. This preventsfeedback into the broadband source 112 that might cause lasing orotherwise change, e.g. produce ripple in, the emission spectrum of thebroadband optical signal from the broadband source 112. If the source112 starts lasing on the broadband reflection from the tunable filter150, the lasing will occur near the spectral gain peak of the source andwill not tune spectrally with the tunable filter tuning, thus destroyingthe intended and required swept frequency operation of the source.

The tunable optical signal 154 that is produced by the passband of theself-tracking tunable filter 150 is transmitted to a secondnon-reciprocal, Faraday, rotator 170. The orientation of the secondFaraday rotator 170 is reverse to the first Faraday rotator 118. As aresult, the polarization of the tunable optical signal is rotated by 45degrees.

The tunable optical signal is then transmitted through a secondpolarizer 172. The orientation of the second polarizer transmits lighthaving a polarization that is rotated 45 degrees with respect tohorizontal. A subsequent halfwave plate 212 rotates the polarizationback to horizontal. As a result, the tunable signal from the secondFaraday rotator 170 is substantially transmitted to an optical amplifier174.

Preferably the optical amplifier 174 in this and the other embodimentsis an SOA with antireflection coated and angled front facet 176,enabling integration onto the bench 110 by attachment, typically via asubmount. The tunable signal is amplified in the semiconductor opticalamplifier 174 in a double pass reflective arrangement. The horizontalpolarization allows standard edge-emitting gain chips be used in astandard flat, planar installation on the bench 110.

In one embodiment, a single angled facet (SAF) SOA is used to amplifythe tunable signal in a first pass through the SOA. Then the reflectedsignal from a reflective back facet is amplified in the SOA for a secondtime with the amplified light exiting the front facet of the SOA chip174.

In the illustrated embodiment, the SOA 174 has antireflection coatedfront facet 176 and back facet 178. Light exiting from the back facet178 is coupled to a discrete mirror 180 that reflects the light backinto the back facet 178 for the second pass through the SOA 174. The twocontra-directional light passes through the SOA 174 have light in thesame polarization orientation; thus a single polarization, that ispolarization sensitive, SOA amplifier can be used here.

Light exiting out of the front facet 176 of the SOA 174 is transmittedthrough the halfwave plate 212 and the second polarizer 172 to thesecond Faraday rotator 170. The tunable optical signal passing throughthe tunable filter 150 a second time, now in the direction opposite tothe first passage, ensures that the noise outside the passband of thetunable filter is further attenuated and the linewidth of the tunablesignal is further narrowed.

Light that is rejected or reflected by the self tracking filter 150 isfiltered by the second polarizer 172. In more detail, the secondpolarizer 172 prevents, filters, light returning from the tunable filter150 through the Faraday rotator 170 from reaching to the SOA amplifierchip 174. That is, light reflected by the tunable filter 150, forexample, passes through the Faraday rotator 170 a total of two times andthus is rotated by 90 degrees and thus is absorbed by the secondpolarizer 172. This prevents the formation of a laser cavity, which herewould destroy the device operation.

The polarization of the light from the SOA 174 and passing through thetunable filter 150 has a polarization that is orthogonal to the originalpolarization of the broadband signal 114 generated by the broadbandsource 112. This is due to the double pass of the tunable signal throughthe Faraday rotator 170. As a result, the tunable signal is now at apolarization that is reflected by the PBS 120, perpendicular to theplane of the bench 110. Thus the tunable signal 154 exits as the outputsignal 190.

Preferably, in this embodiment and the other embodiments discussed here,all of the optical components, are connected to a single, common opticalbench 110, such as by solder bonding. In the illustrated embodiment,coupling or collimation optics such as micro lenses are not shown in theoptical train for ease of illustration. Generally, lenses are requiredat least at the exit facets of the semiconductor chips, i.e, broadbandsource 112 and SOA 174, and to couple light into and out of the tunablefilter 150. In more detail, with respect to the present embodiment,broadband source 112, first polarizer 116, the first halfwave plate 210,first Faraday rotator 118, PBS 120, tunable filter 150, second Faradayrotator 170, second polarizer 172, second halfwave plate 212, SOA 174,and mirror 180 are connected to a common bench 110.

Alternatively, in other implementations, two or more benches are used.

In the illustrated example, the two halfwave plates 210, 212 are notstrictly necessary. If removed, the beams would have a 45 degreepolarization at the PBS 120. To accommodate such polarizations, however,non-standard optical components are required, which could impact theoverall cost and performance.

FIG. 18 illustrates the relationship between the spectral extent of thebroadband optical signal, the scanband and the tunable output opticalsignal 190. In more detail, the broadband source 112 emits or producesuseful levels of optical energy over a spectral range 1810. The passbandof the tunable filter 150 is then tuned over the scanband 1820 thatcovers the desired spectral scanning region of the broadband source'srange 1810. This process produces the narrowband tunable output opticalsignal 190 that is then tuned over that scanband 1820.

FIG. 2 shows a swept optical source 100 b that is a variant of the FIG.1 embodiment swept optical source 100 a, also incorporating acontra-directional self tracking filter using polarization diversity anddouble pass reflective amplification stage.

Generally, the description of the preceding swept optical source 100 aapplies to this embodiment 100 b, with some exceptions.

The light 114 generated by the broadband source 112 passes through anisolator 310. This is a polarization preserving isolator. Thus in theillustrated embodiment, the horizontal polarization light P emitted bythe broadband source 112 exits the isolator 310 with that same parallelpolarization.

The isolator 310 replaces the first polarizer 210, the half wave plate116 and the first Faraday rotator 118 of the FIG. 1 embodiment.

The PBS 120 is oriented to transmit light in the horizontalpolarization. This parallel light passes through the tunable filter 150.In the Faraday rotator 170, light is rotated to a 45° angle. Thepolarizer 172 is oriented to pass light at the 45° polarization angle.In the subsequent halfwave plate 212, the light is rotated to ahorizontal polarization that is amplified in the SOA optical amplifier174. Light returning from the SOA 174 passes through the halfwave plate212, polarizer 172 and the Faraday rotator 170 and is now polarized in adirection that is vertical to the bench 110 and is thus reflected by thePBS 120 as the output signal 190.

FIG. 3 is a scale view of a swept optical source 100 c with aself-tracking filter using polarization diversity that is closelyrelated to the FIG. 2 embodiment swept optical source 100 b.

This figure is useful in showing an actual implementation of a sweptsource on the bench 110. It shows the coupling optics, lenses, that areused to couple the optical signal in and out of the elements on thebench.

In more detail, the broadband source 112 is implemented as a SLED or aSOA in a SLED implementation. The source 112 is installed on a sub mount508. The sub mount in turn is bonded to the bench 110. Typically solderbonding is used between the source chip 112 and the submount 508, andbetween the submount 508 and the bench 110.

Light exiting the back facet of the SOA 112 is typically lost, orcaptured by a beam dump, or constitutes stray light within the hermeticbutterfly package 108 containing the bench 110.

Light exiting the front facet of the SOA 112 is collimated by a firstlens structure 510. In the preferred embodiment, the lens structure 510,and the other lens structures in the source, comprises a mountingstructure 512 that is solder bonded to the bench 110. A lens substrate515 is bonded to the mounting structure 512. Preferably, the mountingstructure is deformable to enable post installation alignment. That is,after solder bonding to the bench 110 the mounting structured 512 isdeformed to ensure that the lens substrate 515 is aligned with respectto the optical axis of the optical signal that is transmitted throughthe lens substrate. The technology is generally described in U.S. Pat.No. 6,416,937 B1.

A collimated broadband signal 114 from the SOA 112 is coupled into anisolator 310. This prevents back reflections in the SOA 112 to preventlasing. In the illustrated embodiment, the isolator 310 is a semi doublestage isolator. It comprises a series of elements: a polarizer followedby a Faraday rotator followed by a polarizer followed by a Faradayrotator and followed by a final polarizer. A final halfwave plate 540 isadded to the back of the polarizer to rotate the broadband signal backto a horizontal polarization.

Light exiting the isolator 310 is coupled into a PBS 120. PBS isconfigured to transmit light that having the polarization of thebroadband source 112. In a typical embodiment, the PBS transmits apolarization that is horizontal with respect to the surface of the bench110 and reflects light at a polarization that is vertical to the surfaceof the bench 110.

The light transmitted through the PBS 120 is coupled to a tunable filter150. This converts the broadband signal 114 into a narrowband tunablesignal 154.

A non reciprocal rotator component 170-1 receives the narrowband tunablesignal 154 from the tunable filter 150. This rotates the polarization45°. The rotator 170 also includes a polarizer. In more detail, rotator170 includes a Faraday rotator 542, a polarizer 544, and a halfwaveplate 546, which rotates the polarization back to input polarization.

The light exiting the rotator component 170-1 is collimated by a secondlens component 516 and coupled into the front facet 176 of the SOAamplifier chip 174. As described previously, a submount 518 connects theSOA 174 to the bench 110. Light exiting the back facet 178 of the SOA174 is collimated by a third lens component 515, reflected by the mirror180, and coupled back into the back facet 178 of the SOA 174. Thus, thetunable signal is amplified in the SOA in a double pass reflectivearrangement.

The light exiting the front facet 176 of the SOA 174 is collimated bythe second lens component 516 and transmitted through the rotatorcomponent 170-1. This rotates the polarization of the tunable opticalsignal 154 such that after it is filtered by the tunable filter 150 thesecond time, the optical signal 154 is reflected by the PBS 120, whichis vertical in the current embodiment. The two passages of light throughthe tunable filter are in the opposite directions and with orthogonalpolarizations.

The vertically polarized light that is reflected by the tunable filter150 as being outside of the passband is absorbed by the polarizer 544 inthe rotator component 170-1.

In the illustrated example, the optical path for the output signal 190is folded to yield a compact design. In more detail, the output signal190, from the PBS 120, is reflected by a fold mirror 520 so that theoutput optical signal is again heading in a direction that is parallelto the broadband signal 114. The output signal is collimated by a fourthlens component 522 and then focused by a fifth lens component 524 ontothe entrance facet of an optical fiber 528. The optical fiber 528 issecured to the bench 110 via a fiber mounting structure 526. Preferably,this mounting structure is also deformable after solder bonding to theoptical bench 110 to enable posts installation alignment of the entrancefacet relative to the surface of the optical bench 110. The opticalfiber exits the hermetic package 108 via a fiber feedthrough. Bothsingle mode fibers and polarization maintaining fibers, as well assingle polarization fibers, are used in different implementations forthe device output fiber 528.

The embodiments described with respect to FIGS. 1 through 3 share acommon characteristic in terms of the SOA or optical amplifier 174. Adouble pass arrangement is used in which the tunable signal is reflectedto pass through the SOA 174 twice. As described, with respect to theembodiments, this can be achieved by using a discrete reflector such asthe mirror (180) or an SAF SOA chip that has an integrated reflector,which is used in other examples.

Under current technology, however, a problem arises with respect to theisolation that can be obtained for the SOA 174. In this double passconfiguration, nearly twice as much isolation is required due to thegain provided by the chip. For example, current chips offerapproximately 30 dB of gain. As a result, at least 60 dB of isolation isrequired to prevent lasing. Moreover, in many instances, the systemfunctions over a tuning range of 100 nm or more. Faraday rotators andhalf wave plates functioning over such a bandwidth typically cannotprovide sufficient isolation. This is typically due to the fact that thehalfwave plate and Faraday rotators only function adequately for thespecified center wavelengths and not over the entire scan band. Thepolarization crosstalk can lead to lasing especially near the edges ofthe scanband.

FIG. 4 shows a swept optical source 100 d with a contra-directional selftracking filter using polarization diversity with a loop amplificationstage 602.

In more detail, the broadband source 112 generates polarized light thatis polarized horizontally with respect to the bench 110. A subsequentisolator 310 prevents back reflections into the broadband source 112.

In the preferred embodiment the broadband source 112 can be an SLED, oran SOA functioning in an SLED configuration or other broadband sourcesuch as those mentioned previously.

In the current embodiment, the broadband source 112 produceshorizontally polarized light, the polarization of which is unchanged bythe isolator 310.

A PBS 120 transmits the polarized light from the source 112, typicallyhorizontally polarized light, and reflects oppositely polarized light.Thus, the broadband signal 114 is coupled into the tunable filter 150,which converts the broadband signal 114 into the narrowband tunablesignal 154. The light is transmitted through a loop PBS 610. A firstloop isolator 612 provides isolation for a subsequent amplifier, or SOA,174. In the preferred embodiment, the first loop isolator 612 preservesthe polarization of the input light.

Two fold mirrors 614 and 616 redirect the tunable signal 154. A secondloop isolator 618 is also provided in the loop 602. A third fold mirror620 turns the tunable optical signal 154 to the loop PBS 610.

The second loop isolator 618 rotates the polarization of the tunablesignal from a direction that is horizontal with respect to the surfaceof the bench 110 to a vertical polarization. As a result, the loop PBS610 reflects the tunable signal back, i.e. to pass through the tunablefilter 150 a second time in the opposite direction and with orthogonalpolarization to the first pass.

In the preferred embodiment, a polarizer or polarizing filter 622 islocated between the loop PBS 610 and the first loop isolator 612. Thispolarizer 622 prevents reflections of the tunable filter 150 frominducing lasing in the loop 602. So in the illustrated embodiment, thepolarizer blocks vertically polarized light.

The tunable optical signal 154 that is transmitted through the tunablefilter 150 now has a polarization that is vertical with respect to thebench 110. As a result, it is reflected by the PBS 120 as the outputsignal 190.

In this embodiment, the transit time of the optical signal through theloop 602 is relevant to the proper operation of the swept source 100 d.If the loop is relatively long in optical distance and the tuning speedof the tunable filter 150 is high and/or the bandwidth of the tunablefilter's passband is narrow, it could arise that the tunable filter 150shifts between the time that the tunable signal 154 is transmitted intothe loop 602 and the time that the tunable signal exits from the loopand passes to the tunable filter 150 as the output signal 190. In thiscase, the tunable signal will be reflected and the output power will below. This fact necessitates an integrated small-time-delay design withlittle or no optical fiber in the loop 602. In fact, in the preferredembodiment, no optical fiber is used in the entire loop and the systemis implemented on a micro optical bench 110 as illustrated.

In one potential modification, the passband of the tunable filter 150 isconfigured to be polarization dependent, for ultrahigh speed tuning Inmore detail, the passband of one polarization is shifted spectrally withrespect to the other polarization. This allows the broadband signallight to enter at the first polarization, then the filter tunes by anincrement corresponding to the shift between the polarization-dependentpassbands and then the tunable signal passes through the tunable filter.By matching the spectral passband shift for the different polarizations,optical delay of the loop 602, and the tuning speed, higher speed tuningcan obtained. On the downside, up/down wavelength tuning would probablynot be possible.

FIGS. 5A and 5B are scale views of a swept optical source 100 e that isclosely related to the FIG. 4 embodiment swept optical source 100 d.

These figures are useful in showing an actual implementation of a sweptsource with contra-directional self tracking filter using polarizationdiversity and a loop amplification stage. They show the coupling optics,lenses, that are used to couple the optical signal in and out of theelements on the bench and the installation of the bench in a hermeticpackage 108. In FIG. 5B, the lid of the package 108 is removed to exposethe optical elements.

The broadband source 112 generates the broadband signal 114. In theillustrated embodiment, the broadband source 112 is implemented as asemiconductor gain chip such as a SLED or SOA implemented as a SLED. Inthe illustrated example, the semiconductor gain chip is secured to asubmount 710, which is bonded to the bench 110. The light exiting fromthe broadband source is collimated by a first lens component 712. Asdescribed previously, the lens components preferably comprise lenssubstrates that are bonded to mounting structures, which in turn aremounted to the bench.

The broadband signal then is transmitted through an isolator 310. Thisprevents back reflections into the broadband source 112 and thus lasing.

The horizontally polarized light from the broadband source 112 istransmitted through the PBS 120. The tunable filter 150, as describedpreviously, then converts the broadband signal 114 into the narrowbandtunable signal 154. The loop PBS 610 transmits the tunable signal 154 toa first loop isolator 612. A second lens component 720 couples thetunable optical signal 154 into the SOA 174 which is the opticalamplifier in the loop 602. As described previously, the SOA 174 ispreferably mounted onto the bench 110 by a submount 725.

Light exiting the SOA 174 is collimated by a third lens component 722 inthe loop 602. Two subsequent fold mirrors 614 and 616 redirect thetunable optical signal. The tunable optical signal is then transmittedthrough a second loop isolator 618 and two lens components: a fourthlens component 724 and a fifth lens component 726. A fold mirror 620returns the tunable optical signal to the loop PBS 610. As describedpreviously, the second loop isolator 618 rotates the polarization of thetunable optical signal by 90° from horizontal to vertical polarization.As a result, the tunable optical signal received by the loop PBS 610 isreflected back to the tunable filter 150.

The tunable optical filter 150 again filters the tunable optical signal154 applying its bandpass filter function. In passing through thetunable filter 150 this second time, light is propagating in theopposite direction and with orthogonal polarization to the firstpassage. The vertical polarization of the tunable optical signal fromthe tunable filter is reflected by the PBS 120 as the output signal 190.

The output signal path is folded to yield a compact design. In moredetail, a fold mirror 730 reflects the output signal 190 to a directionparallel to the original broadband signal 114. A sixth lens component738 focuses the light onto the entrance facet of an optical fiber 528.The fiber entrance facet is secured to the optical bench 110 via a fibermounting structure 740.

FIG. 6 shows a swept optical source 100 f that is a variant of the FIG.4 embodiment swept optical source 100 d.

Generally, the description of the preceding swept optical source 100 dapplies to this embodiment 100 f, with some exceptions.

The swept source 100 f differs from the swept source of FIG. 4 in thatit adds a second amplification stage in the loop 602. In more detail, athird isolator 810 is added to the loop 602. In the illustrated example,the third isolator 810 is located after the SOA 174 and between the foldmirrors 614 and 616. A second loop SOA 812 follows the third isolator810. Light from the second loop SOA 812 is then transmitted through thesecond isolator 618 to the loop PBS 610.

The addition of the second amplification stage improves the power of thetunable optical signal that is generated by the swept source 100 fenabling applications requiring even higher power optical signals.

FIG. 7 shows a swept optical source 100 g that is a variant of the FIG.6 embodiment swept optical source 100 f.

Generally, the description of the preceding swept optical source 100 fapplies to this embodiment 100 g, but this embodiment includesadditional components for potentially higher performance and addedcapabilities.

In this embodiment, a transmissive tracking filter 910 is added betweenthe first loop SOA 174 and the second loop SOA 812. In more detail, thetunable signal exiting from SOA 174 is reflected by fold mirror 614 tothe third isolator 810. This prevents back reflections in to the SOA 174from a subsequent tracking filter 910. This tracking filter 910 has atunable passband similar to tunable filter 150. It is tuned by a tuningcontroller 125, along with the self tracking filter 150 so that itspassband corresponds to the spectral position of the passband of thetunable filter 150. In short, during swept source operation, thetransmissive tracking filter 910 is tuned synchronously with tunablefilter 150 by the controller 125.

The tracking tunable filter 910 removes any noise outside of the signalband of the tunable signal 154. For example, it suppresses any ASEemissions from the SOA 174. The tunable signal from the transmissivetracking filter 910 is then transmitted through a fourth isolator 912.This prevents the subsequent SOA 812 from lasing.

FIG. 8 shows a swept optical source 100 h that is a variant of the FIG.4 embodiment swept optical source 100 d.

Generally, the description of the preceding swept optical source 100 dof FIG. 4 applies to this embodiment 100 h, but this embodiment includesadditional components for potentially higher performance and addedcapabilities.

The swept source 100 h differs from the FIG. 4 embodiment in itsaddition of an output amplification stage. In more detail, the outputtunable optical signal 190 from the PBS 120 is preferably redirected bya fold mirror 1005. The output tunable optical signal 190 is thentransmitted to an output stage isolator 1010. From the isolator, theoutput signal 190 is coupled into an output optical amplifier 1012,preferably an SOA. However, in other implementations, other amplifiertechnologies are used such as rare-earth doped (e.g., Er-doped) opticalfiber amplifiers.

In one particular implementation, the output stage isolator 1012 rotatesthe polarization of the output optical signal 190 from a verticalpolarization to a horizontal polarization, which is the preferredpolarization for many commercially available wideband SOA's.

In operation, once the signal is sufficiently amplified (typically −15dBm), the final stage amplifier 1012 is driven to saturation. In thismode of operation, the gain outside the filter's passband isdramatically reduced, thus reducing the ASE contribution outside thefilter's passband. It should be noted that the first amplifer SOA 174 isconfigured differently with respect to the output gain SOA 1012. Thefirst SOA 174 is operated in a high gain configuration whereas theoutput SOA 1012 is operated in a high saturation regime to lower ASEoutput.

FIG. 9 shows another embodiment of the swept source 100i with aco-directional self tracking tunable filter using polarization diversityand a loop amplification stage, which has been constructed according tothe principles of the invention.

The broadband source 112 is used to generate the broadband ASE signal114. In the preferred embodiment, source 112 is a SLED or SOAimplemented in a SLED configuration.

In one particular example, an SOA is used along with a power boostingreflector 1105, which is added opposite to the back facet 1108 of theSOA 112. The function of the power boosting reflector 1105 is to reflectback to the SOA the ASE light that is emitted out of the back facet1108. Without this reflector, this ASE light is lost to the device andbecomes stray light inside the device package. Such power boostingreflector serves to increase the output power of the broadband signal114 that is generated by the chip. In many instances, however, it hasthe deleterious effect of decreasing the spectral band or spectralextent of the broadband signal 114. As such, the power boostingreflector 1105 component can be added to any of the SOA broadbandsources in the other embodiments, when increased power in the broadbandsignal 112 is desired. On the other hand, when a broadband signal thatincludes a larger spectral band is required, the power boostingreflector 1108 is removed. Note that such double-pass SOA amplifierarrangement still operates to produce only amplified spontaneousemission and does not enter lasing regime, as the single reflector nextto the amplifier does not form an optical resonator with periodicfeedback that would be required to achieve lasing.

The broadband signal 114 is transmitted through an isolator 310 to blockback reflections into the SOA 112. This prevents the formation of alaser cavity. The broadband signal 114 is then transmitted through apolarization beam splitter 1110. In a typical embodiment, the lightemitted by the SOA 112 has a horizontal polarization, and the PBS 1110is configured to transmit this horizontal polarization.

The broadband signal 114 from the PBS 1110 is transmitted in a firstpass through the self tracking tunable filter 150. This converts thebroadband signal 114 into the narrowband tunable signal 154. Asubsequent isolator 1112 prevents back reflections from the tunablefilter 150 from reaching a subsequent amplifier 174 in the loop 1130,which is implemented as a polarization independent SOA. This SOA 174 ispolarization independent in that it preferably provides equal gain tooptical signals along both polarizations, i.e., both parallel andvertical polarizations.

The light emitted by the SOA 174 is reflected by a first fold mirror1114 and a second fold mirror 1116. The tunable signal 154 is thentransmitted through a second loop isolator 1118 and then transmittedthrough a half wave plate 1120. The halfwave plate rotates thepolarization of the tunable signal 154 by 90°. Thus, in one specificexample, the tunable signal that was originally horizontally polarized,is now polarized in the vertical direction after transmission throughthe half wave plate 1120.

A second PBS 1122 is configured to reflect the tunable optical signalduring its first transmission over the loop 1130. Thus in oneembodiment, the PBS 1122 is configured to reflect light in the verticalpolarization.

This vertically polarized light is returned to the first PBS 1110 and isthus reflected by it to pass through the tunable filter 150, in a secondco-directional pass with orthogonal polarization, followed by theisolator 1112, the SOA 174 and the second loop isolator 1118 again. Onthe second transit through the half wave plate 1120, the tunable opticalsignal is now rotated to a horizontal polarization that is transmittedthrough the PBS 1122 to appear as the output signal 190.

The embodiment of FIG. 9 (100 i) differs from other embodiments in thatits loop 1130 supports the transmission of two polarization modes in theloop simultaneously. This configuration advantageously uses the tunablefilter 150 in a co-directional self tracking configuration, while alsoin effect creating two gain stages out of the single SOA 174.

FIG. 10 shows another embodiment of the swept source 100 j. In terms ofconfiguration, it shares many similarities with the embodiment of theswept source 100 i that is illustrated in FIG. 9. It differs in that thetunable optical signal only transits the loop a single time in thisembodiment. This lowers the potential gain provided by the loop andremoves the necessity for a polarization independent amplification inthe loop.

In more detail, as discussed in the with respect to the previousembodiment, the broadband signal 114 is generated in the broadbandsource 112 and transmitted through the isolator 310 and then transmittedthrough a PBS 1110 to the tunable filter 150. After the tunable filter,the now narrowband tunable signal 154 is transmitted to a second PBS1212. A subsequent first loop isolator 1112 prevents reflections intothe subsequent SOA 174, which provides amplification in the loop 1210.The light output from the SOA 174 is reflected by two fold mirrors 1114and 1116 and then is transmitted through a second loop isolator 1118.Again, this second loop isolator 1118 prevents back reflections into theSOA 174 to thereby prevent lasing.

A subsequent halfwave plate 1120 rotates the polarization of the tunableoptical signal 154 in the loop 1210 by 90°. Thus, in one configuration,the tunable optical signal 154, which was originally in horizontalpolarization, is now in a vertical polarization.

A subsequent fold mirror 1216 redirects the tunable optical signal tothe first PBS 1110. The rotated polarization of the tunable opticalsignal is now reflected by the first PBS 1110 and again filtered by thetunable filter 150, in a second co-directional pass with orthogonalpolarization, and then reflected by the second PBS 1212. Thus, thetunable optical signal 154 transits the loop 1210 on a single timebefore it appears as the output signal 190. Output signal 190 emergesright after passing through the tunable filter 150; this reduces theamount of undesired residual broadband spontaneous emission in theoutput signal.

FIG. 11 shows another embodiment of the swept source 100 k that uses anSOA in a double pass reflective configuration and also incorporates aloop surrounding the co-directional self tracking tunable filter 150.

In more detail, the broadband signal is generated by the broadbandsource 112 and is transmitted through an isolator 310 to a PBS 1110. PBS110 is configured to pass the polarization of the SOA 112, typically thehorizontal polarization. The broadband signal 114 is then transmitted ina first pass through the tunable filter 150 to generate the tunableoptical signal 154. A subsequent isolator 1308 prevents backreflections. Then, the tunable optical signal 154 is transmitted througha second PBS 1310.

The tunable optical signal then enters a double pass gain stage.Specifically, a Faraday rotator 1312 rotates the polarization by 45°,then a subsequent halfwave plate 1314 rotates the polarization of thetunable optical signal by 45° in the opposite direction to produce, forexample, horizontal polarization convenient for SOA 174. The resultingpolarization is transmitted through a polarizer 1316 to be amplified inthe amplifier or SOA 174. A reflector 180 at or adjacent to the outputfacet of the SOA 174 reflects the optical signal to pass through the SOA174 again for further amplification. Polarization dependent, i.e. singlepolarization, SOA is preferably used in this reflective amplifierconfiguration. Since the polarization of the optical signal was notrotated in the SOA, it is again transmitted through the polarizer 1316.The subsequent halfwave plate 1314 rotates the polarizationreciprocally. And the polarization is again rotated non-reciprocally by45° in the Faraday rotator 1312 to polarization orthogonal to that whenit entered the gain stage. Due to the non-reciprocal nature of theFaraday rotator 1312, the returning tunable optical signal is noworthogonally polarized and is reflected by the second PBS 1310 andenters the loop 1318.

A first fold mirror 1320 in the loop 1318 reflects the tunable opticalsignal to pass through an isolator 1322 and then be reflected by asecond fold mirror 1324. This returns the tunable optical signal to thefirst PBS 1110. Since the polarization of the tunable optical signal hasnow been rotated relative to the original polarization of the broadbandsignal 114, to a vertical polarization in one example, the tunableoptical signal 154 is reflected by the PBS 1110 to pass through thetunable filter 150 a second time, co-directional and orthogonallypolarized to the first pass, and then through isolator 1308. Now, withthe rotated polarization, the tunable optical signal 154 is reflected bythe second PBS 1310 to appear as the output signal 190.

FIG. 12 shows a swept source 1001 that is similar to the configuration(100 k) illustrated in FIG. 11. This specific configuration makes moreefficient use of the isolators, being able to achieve similarfunctionality with one less isolator than the FIG. 11 embodiment.

Here, the broadband signal 114 produced by the broadband source 112 iscoupled directly into the PBS 1110 without an intervening isolator.Instead the isolator 310 is moved inside the loop 1410. That is, thebroadband signal 114 is transmitted through the PBS 1110 to the firstisolator 310. Then, the broadband signal 114 is coupled into the tunablefilter 150 to produce, in a first pass, the subsequent narrowbandtunable signal 154. A second isolator 1308 is also located in the loop1410. The tunable signal 154 is transmitted through a second PBS 1310.The tunable signal is then coupled into a double pass reflective gainstage similar to that described with respect to FIG. 13 comprising aFaraday rotator 1312, a halfwave plate 1314, a polarizer 1316, and theSOA 174 in a double pass reflective configuration.

The amplified tunable signal 154 is received back from the double passgain stage in orthogonal polarization due to the non-reciprocaloperation of the Faraday rotator 1312. As a result, the tunable signalis reflected into the loop 1410 by the second PBS 1310 to a pair of foldmirrors 1320 and 1324. This returns the tunable signal to the first PBS1110, where it is reflected. It passes through the isolator 310 and thetunable filter 150 a second time, co-directionally and with orthogonalpolarization to the first pass. After transmission through the secondisolator 1308, the tunable signal 154, with its now orthogonally rotatedpolarization, is reflected by the second PBS 1310 to appear as theoutput signal 190.

FIG. 13 shows another embodiment 100 m that uses the combination of adouble pass loop configuration with a co-directional self-trackingfilter, this followed by a second gain stage.

In more detail, a broadband source 1510 is located within a double passloop 1516. In the preferred embodiment, the broadband source 1510 is asemiconductor gain chip such as a SOA that has anti-reflection coatedfront and back facets. Moreover, the SOA 1510 is selected to bepolarization isotropic or polarization independent. As a result, itamplifies light at either polarization, preferably equally.

The SOA 1510 generates ASE light that serves as the broadband seed 114.The broadband light is transmitted through a first isolator 1512 toprevent lasing. Then, the narrowband tunable signal 154 is generated bythe transmission of the broadband signal through the tunable filter 150.This narrowband signal 154 is then reflected by the PBS 1514, whichselects a single polarization for re-amplification inside the loop. Afirst fold mirror 1518 reflects the tunable signal to pass through asecond isolator 1520 and a halfwave plate 1522. The narrowband signal,now in orthogonal polarization, is then returned to the back facet ofthe SOA 1510 using two more fold mirrors 1524 and 1526.

The tunable optical signal injected through the rear facet of the SOA1510 is amplified by the SOA. It passes through the isolator 1512 andthe tunable filter 150, a second time co-directionally and withorthogonal polarization to the first pass. Due to the operation of thehalfwave plate 1522, the amplified tunable signal is now orthogonallypolarized and is now transmitted through the PBS 1514. Then, tunablesignal 154 passes through a third isolator 1528 and preferably undergoesfurther amplification in an amplifier or SOA 174. A final isolator 1530is preferably used to prevent back reflections and lasing by the SOA174.

In one embodiment, a final tracking tunable filter 1532 is provided.This tracking filter is useful to attenuate ASE emissions outside of thesignal band of the tunable signal 154. The tracking filter 1532 is tunedso that it is passband coincides with that of the tunable filter 150. Inhigh-speed operation, it is tuned synchronously with the tunable filter150.

This embodiment has advantages insofar as it optimally uses the firstSOA 1510. It operates both as an ASE seed and a first amplificationstage.

FIG. 14 shows another embodiment 100p that utilizes two loops 1626 and1640 to support the co-directional self tracking filter 150.

In more detail, a broadband source 1610 functions as an ASE seed.Preferably, the broadband source 1610 is an SOA that has anti-reflectioncoated on its front and rear facets. The ASE light is transmitted out ofthe rear facet of the SOA 1610. A first polarization beam splitter 1612selects to transmit a single polarization of the ASE light produced bythe SOA 1610. The broadband signal 114 is then transmitted through asecond PBS 1614. A subsequent isolator 1616 prevents back reflectionsand otherwise transmits the broadband signal 114 to the self trackingtunable filter 150.

The operation of the filter function on the broadband signal 114produces the narrowband tunable signal 154. A subsequent third PBS 1620is configured to reflect the light at the polarization of the broadbandsignal 114 and the tunable signal 154 on its first pass through thetunable filter 150.

The PBS 1620 diverts this first-pass light to the first loop 1626. Twofold mirrors 1630 and 1634 return the tunable signal to the second PBS1614. A halfwave plate in the first loop 1626 rotates the polarizationby 90°. As a result, on returning to the second PBS 1614, the tunablesignal 154 is reflected to again pass through the isolator 1616 andtunable filter 150, co-directionally and with orthogonal polarization tothe first pass. With its rotated polarization, the tunable signal 154,after the second pass, is transmitted through the third PBS 1620 to thesecond loop 1640. Three fold mirror's 1642, 1644, and 1648 form thesecond loop 1640 and return the tunable signal to the first PBS 1612.And isolator 1646 is preferably placed in the second loop 1642 preventback reflections.

On returning from the second loop 1640, the tunable signal is reflectedby the first PBS 1612 to pass through the SOA 1610, again, now in theopposite direction and with orthogonal polarization. This allows the SOA1610 to function as an amplifier for the tunable signal 154. The outputsignal 190 is then taken from an isolator 1650.

FIG. 15 shows another embodiment 100o which is related to the embodiment(100 n) described with respect to FIG. 14. It is a two loopconfiguration. It adds two stages of amplification, however.

In more detail, as described with respect to FIG. 14, the SOA 1610functions as the ASE seed. The broadband light 114 is transmittedthrough the first PBS 1612 and the second PBS 1614. The broadband signalis then transmitted through the first isolator and the tunable filter150 to produce the narrow band tunable signal 154. A subsequent isolator1710 is added along with an amplifier or SOA 1711. Preferably this firstSOA, amplification stage, is polarization isotropic. As a result, itamplifies light along either polarization. The third PBS 1620 reflectsthe amplified tunable signal to an isolator 1712 that prevents backrecollections into the SOA 1711. The two fold mirrors 1630 and 1634 formthe loop 1708 that returns the amplified tunable signal to the secondPBS 1614.

Preferably the first loop 1708 further includes a polarizer 1714, ahalfwave plate 1632 and a second SOA amplification stage 1716. Thesecond SOA application stage 1716 can be polarization anisotropic. Thatis, it amplifies light predominantly only along a single polarization,such as vertical. One can also reverse the order of the half wave plate1632 and the SOA 1716, with the optical beam in the loop 1708 firstpassing through the SOA 1716 and then the polarization rotating halfwaveplate 1632. In this case the SOA 1716 can operate with horizontal gainpolarization. As described in the previous embodiment, the halfwaveplate 1632 in the first loop 1708 rotates the polarization so that thelight returning to the second PBS 1614 is reflected to pass againthrough the isolator 1616, tunable filter 150, isolator 1710, and firstamplification stage SOA 1711.

The rotated polarization of the now amplified tunable signal 154 meansthat the amplified tunable signal is transmitted through the third PBS1620 to the second loop 1640. As described in the previous embodiment,the second loop 1640 returns the tunable signal to the first PBS 1612.Now, the tunable signal 154 is transmitted again through the seed SOA1610. A final isolator 1650 on the output path yields the output signal190 while preventing lasing in the seed SOA 1610.

In total, the embodiment of FIG. 15 provides four stages ofamplification. This is achieved with only three optical amplifiers.

In other embodiments, either the first SOA amplification stage 1711and/or the second SOA amplification stage 1716 are eliminated if thefull four stage amplification is not required. With such modifications,some of the additional isolators are eliminated.

FIG. 16 shows an optical coherence analysis system 300 using the sweptsources 100 described above.

In more detail, a Michelson interferometer 1610 is used to analyze theoptical signals from the sample 340. The tunable output optical signal190 from the swept source module 100 is output on fiber 528 to a, forexample, 90/10 optical coupler 322. The tunable signal 190 is divided bythe coupler 322 between a reference arm 326 and a sample arm 1618 of thesystem. The optical fiber of the reference arm 326 terminates at thefiber endface 328. The light exiting from the reference arm fiberendface 328 is collimated by a lens 330 and then reflected by a mirror332 to return back.

The external mirror 332 has an adjustable fiber to mirror distance (seearrow 334). This distance determines the depth range being imaged, i.e.the position in the sample 340 of the zero path length differencebetween the reference arm 326 and the sample arm 1618. The distance isadjusted for different sampling probes and/or imaged samples. Lightreturning from the reference mirror 332 is returned to a reference armcirculator 342 and directed to a 50/50 fiber coupler 346.

The fiber on the sample arm 1618 terminates at the sample arm probe 336.The exiting light is focused by the probe 336 onto the sample 340. Lightreturning from the sample 340 is returned to a sample arm circulator 341and directed to the 50/50 fiber coupler 346. The reference arm signaland the sample arm signal are combined in the fiber coupler 346. Thecombined/interference signal is detected by a balanced receiver,comprising two detectors 348, at each of the outputs of the fibercoupler 346. The electronic interference signal from the balancedreceiver 348 is amplified by amplifier 350.

An analog to digital converter system 315 is used to sample theinterference signal output from the amplifier 350. Frequency clock andsweep trigger signals derived from the swept source are used by the A2Dboard 315 to synchronize system data acquisition with the frequencytuning of the swept source.

Once a complete data set has been collected from the sample 340 byspatially raster scanning the focused probe beam point over the sample,in a Cartesian geometry x-y fashion or a cylindrical geometry theta-zfashion, and the spectral response at each one of these points isgenerated from the frequency tuning of the swept source 100, the digitalsignal processor 380 performs a Fourier transform on the data in orderto reconstruct the image and perform a 2D or 3D tomographicreconstruction of the sample 340. This information generated by thedigital signal processor 380 can then be displayed on a video monitor.

FIG. 17 shows another application, for example spectroscopicapplication, for the swept sources 100. In this embodiment, the sweptsource 100 generates the narrowband tunable output signal 190. This istransmitted on an optical fiber 528 to a probe 380. A sample 340 isilluminated by the tunable signal 190 from the probe 380. A detector 382detects the diffuse or specular reflectance, typically, from the sample340. By tuning the swept source 100 over the wavelength scan band, thetime-resolved response of the detector 382 corresponds to the spectralresponse of the sample 340. In this way, the swept source is used in aspectroscopy analysis system 384.

One advantage of the swept source 100 is that it has a widelycontrollable level of coherence, since it is not a laser. This isimportant for controlling and limiting speckle, which can undermine theaccuracy of the spectral analysis. Tunable signals with a wider dynamiclinewidth, such as possible with the filtered ASE swept sources, willhave lower measured speckle levels than the typically very narrowlinewidth swept laser sources.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A swept optical source, comprising: a tunablefilter for spectrally filtering light from a broadband source togenerate a tunable optical signal that is spectrally tuned over ascanband; a semiconductor optical amplifier for amplifying the tunableoptical signal with the amplified tunable optical signal being againfiltered by the tunable filter before exiting the swept optical sourcewithout again being amplified by the semiconductor optical amplifier toprevent lasing and formation of a resonator; and a tuning controllerthat drives the tunable filter to tune over the scanband at speedsgreater than 10 kiloHertz.
 2. A swept source as claimed in claim 1,wherein the broadband source generates light over the scanband.
 3. Aswept source as claimed in claim 1, wherein the broadband sourcecomprises an amplified spontaneous emission source that generates lightover the scanband.
 4. A swept source as claimed in claim 1, wherein thetunable filter is a Fabry Perot tunable filter.
 5. A swept source asclaimed in claim 1, wherein the tunable filter is a micro electromechanical system Fabry Perot tunable filter.
 6. A swept source asclaimed in claim 1, wherein in a passband of the tunable optical filteris less than 20 GigaHertz (FWHM).
 7. A swept source as claimed in claim1, wherein in a passband of the tunable optical filter is less than 5GigaHertz (FWHM).
 8. A swept source as claimed in claim 1, furthercomprising a tuning controller that drives the tunable filter to tuneover the scanband at speeds greater than 100 kiloHertz.
 9. A sweptsource as claimed in claim 1, further comprising a reflector forreflecting the tunable optical signal to pass through the opticalamplifier a second time and back to the tunable filter.
 10. A sweptsource as claimed in claim 1, further comprising a loop including theoptical amplifier for directing the tunable optical signal back to thetunable filter.
 11. A swept source as claimed in claim 1, furthercomprising a polarization rotation system for rotating a polarization ofthe tunable optical signal between being filtered by the tunable filter.12. A swept source as claimed in claim 11, wherein the polarizationrotation system includes a non-reciprocal polarization rotation element.13. A swept source as claimed in claim 11, wherein the polarizationrotation system includes a halfwave plate.
 14. A swept source as claimedin claim 1, wherein the filtering by the tunable filter of the tunableoptical signal is co-directional with the filtering of the amplifiedtunable optical signal by the tunable filter.
 15. A swept source asclaimed in claim 1, wherein the filtering by the tunable filter of thetunable optical signal is contra-directional with the filtering of theamplified tunable optical signal by the tunable filter.
 16. A method forgenerating a tunable optical signal, comprising: spectrally filteringlight to generate a tunable optical signal with a tunable filter that isspectrally tuned over a scanband; amplifying the tunable optical signalto generate an amplified tunable optical signal; spectrally filteringthe amplified tunable optical signal with the same tunable filter;transmitting the amplified tunable optical signal as an output andpreventing further amplification to prevent lasing and formation of aresonator; and tuning the tunable filter over the scanband at a speedsgreater than 10 kiloHertz.
 17. A method as claimed in claim 16, furthercomprising generating the light with a broadband source that generateslight over the scanband.
 18. A method as claimed in claim 16, whereinthe tunable filter is a Fabry Perot tunable filter.
 19. A method asclaimed in claim 16, further comprising tuning the tunable filter overthe scanband at a speeds greater than 100 kiloHertz.
 20. A method asclaimed in claim 16, further comprising directing the tunable opticalsignal back to the tunable filter using a loop.
 21. A method as claimedin claim 16, further comprising directing the tunable optical signalback to the tunable filter using a reflector.
 22. A tunable amplifier,comprising: a tunable filter for spectrally filtering light to generatea tunable optical signal that is spectrally tunable over a spectralband; a semiconductor optical amplifier for amplifying the tunableoptical signal with the amplified tunable optical signal being filteredby the tunable filter with no more than two passes through the opticalamplifier to prevent lasing and formation of a resonator; and a tuningcontroller that drives the tunable filter to tune over the scanband atspeeds greater than 10 kiloHertz.
 23. A swept optical source,comprising: a tunable filter for spectrally filtering light from abroadband source to generate a tunable optical signal that is spectrallytuned over a scanband; a semiconductor optical amplifier for amplifyingthe tunable optical signal in a first pass; a reflector for reflectingthe tunable optical signal to pass through the optical amplifier in aonly second pass to prevent lasing and formation of a resonator; and atuning controller that drives the tunable filter to tune over thescanband at speeds greater than 10 kiloHertz.